Model Formulation:Model Formulation

GSSHA is a physically-based, distributed-parameter, structured grid, hydrologic model that simulates the hydrologic response of a watershed subject to given hydrometeorological inputs. The watershed is divided into cells that comprise a uniform finite difference grid. Processes that occur before, during, and after a rainfall event are calculated for each grid cell and then the reponses from individual grid cells are integrated to produce the watershed response. Major components of the model include precipitation distribution, snowfall accumulation and melting, precipitation interception, infiltration, evapo-transpiration, surface water retention, surface runoff routing, channel flow routing, unsaturated zone modeling, saturated groundwater flow, overland sediment erosion, transport and deposition, channel routing of sediments, and constituent fate and transport on the overland and in channels.

During an event, rainfall is spatially and temporally distributed over the watershed. Rainfall may be intercepted by vegetation before reaching the land surface. Once an initial interception demand is reached, a fraction of the precipitation will reach the land surface. Upon reaching the land surface, precipitation may infiltrate due to gravity and capillary forces. Water remaining on the land surface may runoff as two dimensional (2-D) overland flow, after a specfied retention depth representing micro-topography has been reached. This water may eventually enter a stream and be routed to the watershed outlet as one dimensional (1-D) channelized flow. Between precipitation events, soil moisture accounting, evapo-transpiration (ET), and 2-D lateral groundwater flow may be occuring. When precipitation falls in the form of snowfall, the water equivalent volume remains on the land surface and is released as water according to an energy budget calculation.

On the overland flow plane sediment is detached due to rainfall impact and shear stresses due to overland flow. Sediments are routed overland along with the 2-D overland flow. Erosion and deposition continuously occurs on the overland plane as sediments are transported. Sediments may eventually be routed to the stream network where fines (silt and clay) are routed according the advection dispersion equation. Coarse materials are treated as bed load, which is computed according to Yang's method (ref).

Constituents may be assumed to within the soil column or on the land surface. In either case, constituent uptake occurs when water is ponded on the soil surface. Constituents move along in the 2-D overland flow, with reactions occuring as water moves across the watershed. Constituents may ultimately be deposisted into the stream network where they are transported according to the reactive advection dispersion equation.